Some parameters that may regulate the miscibility and stability of mixed lipid-protein monolayers at the air-145 mM NaCl interface were studied employing six glycosphingolipids (acidic or neutral), three different types of proteins (soluble, extrinsic or highly amphipathic) and some phospholipids. The results obtained show that the percentage of the total area occupied by the protein at the interface is an important parameter leading to lateral phase separations; the amount and area contribution of the protein accepted in the film before the components become immiscible increase with the complexity of the polar head group of the glycosphingolipids. The interactions occur with progressive reductions of the intermolecular packing as the polar head group of the glycosphingolipid becomes more complex and this is accompanied by more negative values of the excess free energy of mixing. The lipid component seems to be the major responsible for the reduction in mean molecular area.